An ultrasonic diagnostic apparatus irradiates the inside of a subject with ultrasonic waves from transducers and uses the ultrasonic waves reflected from the inside of the subject to display information concerning the form of an organ or the like or the flow of blood on a display portion. This principle is well known (for example, see Patent Document 1).
FIG. 7A is a diagram showing a configuration of a driving pulse generator disposed in a transmitter for driving a transducer in a conventional ultrasonic diagnostic apparatus. In the driving pulse generator, high-voltage switches SW11 and SW12 are connected in series, and a power terminal of +B [V] and a power terminal of −B [V] are connected to the other ends of the high-voltage switches SW11 and SW12, respectively. A node between the high-voltage switches SW11 and SW12 is connected to the transducer (not shown). Further, each of the high-voltage switches SW11 and SW12 is switch-controlled by a timing controller (not shown).
FIG. 7B is a waveform diagram showing a switching operation of the high-voltage switches SW11 and SW12 and a driving pulse train signal to be supplied to the transducer. A group of driving pulse trains shown in FIG. 7B forms an ultrasonic pulse for use in one scanning.
First, when the high-voltage switch SW11 is turned ON, a voltage of +B [V] is applied to the transducer. Then, when the high-voltage switch SW11 is turned OFF, and the high-voltage switch SW12 is turned ON, a voltage of −B [V] is applied to the transducer. By turning ON/OFF the high-voltage switches alternately in this manner, the driving pulse trains formed of a bipolar waveform of two waves are generated as shown in FIG. 7B.
In order to allow an ultrasonic diagnostic apparatus to have a higher resolution in the lateral direction, ultrasonic beams are preferably as thin as possible. This principle will be described herein for the case of transmission only, although the same applies to the case of reception.
As a well-known method for achieving thin ultrasonic beams, timings of the driving pulse trains to be output from a plurality of the transmitters connected to a plurality of the transducers are adjusted. It is known that when the ultrasonic pulses having the same amplitude are generated from all the transducers, low directivity is shown also in locations other than a principal axis, which is called a side lobe.
In order to suppress the side lobe, it is known to be effective to reduce the amplitude of the driving pulse to be supplied to the transducer at an end of an aperture, which is called apodization. In order to perform apodization, it is necessary to vary the amplitude of the driving pulse with the driving pulse generator depending on where the corresponding transducer is located in the aperture. Higher variability (number of stages in which the amplitude is varied) is more effective in suppressing the side lobe.
FIG. 8A is a diagram showing a configuration of the driving pulse generator that is capable of varying the amplitude of the driving pulse. FIG. 8B is a waveform diagram showing switching of high-voltage switches SW13 to SW16 and a driving pulse train signal. As shown in FIG. 8A, the driving pulse generator is configured so that a plurality of positive power terminals and a plurality of negative power terminals are connected to the transducer via the respective high-voltage switches SW13 to SW16. As shown in FIG. 8B, in accordance with the timing of switching the high-voltage switches SW13 to SW16, driving pulse trains having an amplitude of 2B2 [V] and driving pulse trains having an amplitude of 2B1 [V] are generated.